This invention relates to a diversity transmission method and a base station for a mobile radio system for carrying out the method.
In a document entitled xe2x80x9cUTRA FDD; Physical layer proceduresxe2x80x9d and designated xe2x80x9c3GPP PAN S1.14 V2.0.0xe2x80x9d, which was published by the standardization organization 3GPP (Third Generation Partnership Project) in April 1999 within the scope of the standardization of the future mobile radio system UMTS (universal mobile telecommunications system), and which can be accessed in the Internet at the home page xe2x80x9chttp://www.3gpp.orgxe2x80x9d, a diversity transmission method for a mobile radio system and a base station for carrying out the method are described. In Chapter 8 on pages 23 to 25, entitled xe2x80x9cFeedback mode transmit diversityxe2x80x9d, a base station is described which transmits a first signal and a second signal via two spaced antennas. The signals differ in their pilot sequences, i.e., in their identifications, which are used to mark the two antenna paths and, thus, the two radio transmission paths. Thus, at the receiving location, an amplitude difference and a phase difference between the two signals can be determined. The mobile station at the receiving location sends a feedback signal to the base station to indicate the values of the existing amplitude and phase differences, so that the amplitude and phase can be corrected at the transmitting location. Since only a single feedback signal is transmitted, correction is only possible between two antennas. It is desirable, however, to provide a diversity transmission method for more than two antennas and to propose a base station for carrying out the method.
U.S. Pat. No. 5,652,764 discloses a mobile communication system in which a base station transmits radio signals via two spaced antennas by the diversity technique. A transmission signal is spread through two different orthogonal codes to produce two different spread signals, which are then transmitted via the two spaced antennas. The mobile station receiving these signals can identify the signals by the codes. The codes are thus identifications; they are generated in the base station by two transmitting means provided with code generators. Accordingly, the base station described in that U.S. Patent includes a first transmitting means and a second transmitting means which form from a transmission signal a first signal with a first identification and a second signal with a second identification. The mobile station, which can distinguish the two signals from one another by the different identifications, can also determine an amplitude difference and a phase difference existing between the two signals at the receiver. On reception, a constructive combination of the two signals provides a diversity gain, which increases as the amplitude and phase difference decreases. The prior-art diversity transmission method is carried out using two spaced antennas. It is desirable, however, to apply the method to more than two antennas, particularly to antenna arrays and phased arrays.
It is therefore an object of the invention to provide a diversity transmission method for more than two antennas and a base station for carrying out this method.
This object is attained by a diversity transmission method with the features of claim 1 and by a base station with the features of claim 7.
Accordingly, use is made of more than two antennas, the antennas being divided into a first antenna group and a second antenna group in a first step, and the first signal being transmitted via the first antenna group and the second signal via the second antenna group in a second step. In a later step, a new first antenna group and a new second antenna group are formed at least once, and the second step is then repeated. By these measures, the diversity transmission method is applied to several antenna groups each comprising part of the more than two antennas, with two antenna groups being formed several times using different groupings of the antennas, and the process being cycled through several times. As a result, a different amplitude difference and/or phase difference to be corrected appears with each cycle. Through the cyclic repetition of the process, a high diversity gain can be achieved in a simple manner if more than two antennas are used.
The base station according to the invention is characterized in that it is connected to more than two antennas, that it comprises assignment means for assigning each of the antennas to either of the two transmitting means, and that it further comprises computing and control means which are connected to the assignment means, divide the antennas into a first antenna group and a second antenna group, and control the assignment in such a manner that the first antenna group and the second antenna group are connected to the first transmitting means and the second transmitting means, respectively.
Further advantageous features of the invention are defined in the subclaims.
Particularly advantageously, in a third step, the first and second signals are received and an amplitude difference and/or a phase difference existing between the two signals at the receiving location are determined, and in a next step, this amplitude difference and/or this phase difference are communicated to the transmitting location, where the amplitude and/or the phase position are changed for one of the two antenna groups. The base station transmitting the signals thus receives from the mobile station a correction signal which indicates the amplitude difference and/or phase difference existing at the receiver. The base station uses this correction signal to correct the amplitude and/or the phase position, so that the signals arrive at the receiver with equal strengths and in phase and can be constructively combined in order to achieve a high diversity gain.
In this connection it is particularly advantageous to change the amplitude and/or the phase position for one of the two antenna groups by complex weighting of those signals which are transmitted via this antenna group.
If M antennas are used, where M is a power of two, it is particularly advantageous if for the grouping of the M antennas, a Walsh-Hadamard matrix consisting of Mxc3x97M elements is formed, each of whose M columns is assigned to one of the M antennas, and of whose M rows, those Mxe2x88x921 rows which follow the first row each indicate an assignment of the antennas to one of the two antenna groups. By the formation of this matrix, a total of Mxe2x88x921 possible different groupings are calculated in a simple manner, each of which can be used for one cycle of the process for dividing the antennas into the antenna groups. With these Mxe2x88x921 possible groupings, the M antennas can be adjusted relative to each other nearly optimally, since the Walsh-Hadamard matrix provides exactly Mxe2x88x921 orthogonal associations.
It is also particularly advantageous if the method is carried out in a base station which is connected to an antenna array formed from the more than two antennas, and if the base station includes complex weighting stages which are connected to computing and control means and which change the amplitudes and phases of the signals transmitted via the antenna array in response to control signals provided by the computing and control means. A base station so equipped is especially suited for carrying out the diversity transmission method according to the invention on the space or polarization diversity principle.
In this connection it is particularly advantageous to implement the antenna group as a phased array. This makes it possible to carry out the diversity transmission method within an SDMA radio transmission (SDMA=space diversion multiple access) and to serve a large number of users simultaneously by reuse of radio resources.